This invention relates generally to blow pin assemblies used on rotary blow molding machines.
Blow molding machines are used to form hollow, plastic articles such as milk bottles and other liquid containers. The blow molded article is formed in a mold that is constructed from a pair of mold halves which are clamped together around an extruded section of softened thermoplastic resin (referred to as a parison). After clamping, a blow pin is inserted into an opening of the mold and parison and air is then forced through the blow pin into the interior of the parison so that it conforms to the surface of the mold cavity. Movement of the blow pin into and out of the opening in the mold cavity is typically done automatically using a blow pin assembly that operates under PLC control.
On shuttle blow molding machines, the mold is usually moved, or shuttled, between an extrusion work station and a blow pin workstation. At the extrusion workstation, an extruder head extrudes a generally cylindrical or tubular parison between the two halves of an open mold. The mold is then clamped shut around the parison and the mold is transported to the blow pin workstation where a blow pin moves dowmward and into engagement with the opening in the mold cavity, with the blow pin extending slightly into the center of the tube of parison. Air is then blown into the parison to expand it against the sides of the mold cavity.
Sometimes, the rim of the molten parison collapses or gets folded over, thereby preventing the blow pin from entering into the center of the parison. As a result, the parison is not properly expanded in the mold and the resulting article must be rejected. To avoid this problem, the blow pin has sometimes been used to pre-blow the parison; that is, air is forced through the blow pin as it is being brought down into engagement with the mold to help hold the parison open.
On rotary blow molding machines, the molds are located on a carrousel that indexes the molds in a circular path through different workstations. The molds are paused at each workstation for a different operation such as, for example, extrusion of the parison into the mold at an extrusion workstation. On these types of blow molding machines, the blow pin assembly may be mounted on the carrousel so that it indexes with its associated mold. Typically, this is done by mounting the blow pin assembly on the mold clamp so that it moves with one of the two halves of the mold clamp. However, since the blow pins are indexed around through the workstations along with their associated molds, the blow pin assembly must not only move the blow pin into and out of the mold opening, but must also retract the blow pin away from above the opening. Otherwise, the blow pin would interfere with the extrusion head when it dispenses a parison at the extrusion workstation.
As a result, blow pin assemblies for rotary blow molding machines have been designed to move the blow pin along an arcuate path between a retracted, or home, position and an extended or mold-engaging position. See, for example, U.S. Pat. No. 4,919,607, issued Apr. 24, 1990 to M. Warren Martin et al. However, one problem with these types of blow pin assemblies is that, because of the arcuate path, the blow pin does not enter the opening along a purely linear path (i.e., along the central axis of the mold cavity that extends through the opening). While the Martin et al. patent utilizes a pivotal linkage that minimizes the arcuate travel of the blow pin as it approaches and enters the opening in the mold, the movement of the blow pin nonetheless has a horizontal component to it that can result in the blow pin catching the upper edge of the parison and folding it over and at least partially closing the open end of the parison that receives the blow pin. This makes proper expansion of the parison in the mold impossible. Also, with this type of blow pin assembly it may not be possible to pre-blow the parison because the blow pin is not entering from directly above the parison and doing so can again fold over the upper edge of the parison.
Nonetheless, after a parison is provided into the mold and the extruder head is retracted away from the mold, the mold along with its blow pin mounted thereon is indexed a significant distance downstream of the extruder so that the blow pin clears the extruder head and can be inserted into the mold. This significant movement of the mold after extrusion and before insertion of the blow pin can introduce a number of disadvantages. For example, it can increase cycle time and/or reduce the time available to blow pressurized air into the parison, and can cause the parison to sag or shift in the mold. This reduces the efficiency of the blow molding machine, can result in insufficient cooling of the formed part causing post-blowing handling and processing problems and can cause inconsistent wall thickness of the formed part.
Additionally, it is desirable to insert the blow pin and inject pressurized air into the parison as soon as possible after the parison is extruded because the parison becomes less pliable as it cools and hardens. Also, the sooner the blow pin is inserted into the parison, the lower the chance of the parison collapsing or otherwise becoming misaligned within the mold.
Accordingly, there exists the need for a blow pin assembly for a rotary blow molding machine that reduces the possibility of the blow pin catching and folding over the edge of the parison and that permits the parison to be pre-blown to better improve the yield of the machine. Still further, there exists the need for a low. profile blow pin assembly which can be readily inserted into a parison soon after it is extruded and a method for forming a blow molded plastic product wherein the blow pin is inserted substantially at the extrusion workstation of the blow molding machine to avoid the notable and significant disadvantages associated with significant movement of the mold from the extruder workstation prior to insertion of the blow pin and application of the forced air into the parison.
In accordance with the invention there is provided a blow pin assembly which overcomes the above-noted disadvantages of prior art blow pin assemblies used on rotary blow molding machines. The blow pin assembly includes a base, a carriage supported by and movable relative to the base, a sub-base supported by and movable relative to the carriage, and a blow pin mounted to the sub-base for movement therewith. The assembly includes a first actuator that is coupled between the base and the carriage and a second actuator that is coupled between the carriage and sub-base. The first actuator is operable to cause the carriage and sub-base to move relative to the base. The second actuator is operable to cause the sub-base to move relative to the carriage. As a result of activation of the first and second actuators, the blow pin can be reciprocated between a home position and a mold-engaging position with the blow pin moving into and out of the mold-engaging position along a path that is substantially coincident with the central axis that extends through the opening in the mold cavity.
Preferably, the first actuator moves the carriage and sub-base in a direction that is generally perpendicular to the central axis, such that the blow pin moves between the home position and an alignment position in which the blow pin is located directly over the mold opening. The second actuator then moves the sub-base relative to the carriage along the central axis of the opening from the alignment position to the mold-engaging position where the blow pin enters the mold opening.
Preferably, the carriage is slidably received on rails disposed below an upper surface of the mold halves and the blow pin is constructed with a low profile to clear an extruder head above the mold. This permits a method of forming a blow molded product to be used wherein the blow pin is inserted into the mold (and a parison in the mold), while the mold is in a parison receiving position at the extrusion workstation of the blow molding machine or very nearly so.
In accordance with another aspect of the invention there is provided a rotary blow molding machine having a plurality of mold stations, each of which includes a blow pin assembly as described above.
Preferably, each mold station includes a mold clamp with each blow pin assembly being mounted on its associated mold clamp.